Commit 087adb4f authored by Mateusz Guzik's avatar Mateusz Guzik Committed by Christian Brauner
Browse files

vfs: dodge smp_mb in break_lease and break_deleg in the common case 



These inlines show up in the fast path (e.g., in do_dentry_open()) and
induce said full barrier regarding i_flctx access when in most cases the
pointer is NULL.

The pointer can be safely checked before issuing the barrier, dodging it
in most cases as a result.

It is plausible the consume fence would be sufficient, but I don't want
to go audit all callers regarding what they before calling here.

Signed-off-by: default avatarMateusz Guzik <mjguzik@gmail.com>
Link: https://lore.kernel.org/r/20240806172846.886570-1-mjguzik@gmail.com


Reviewed-by: default avatarJeff Layton <jlayton@kernel.org>
Signed-off-by: default avatarChristian Brauner <brauner@kernel.org>
parent 215ab0d8

include/linux/filelock.h

+12 −2
Original line number Diff line number Diff line
@@ -420,28 +420,38 @@ static inline int locks_lock_file_wait(struct file *filp, struct file_lock *fl) 
#ifdef CONFIG_FILE_LOCKING

static inline int break_lease(struct inode *inode, unsigned int mode)

{

	struct file_lock_context *flctx;



	/*

	 * Since this check is lockless, we must ensure that any refcounts

	 * taken are done before checking i_flctx->flc_lease. Otherwise, we

	 * could end up racing with tasks trying to set a new lease on this

	 * file.

	 */

	flctx = READ_ONCE(inode->i_flctx);

	if (!flctx)

		return 0;

	smp_mb();

	if (inode->i_flctx && !list_empty_careful(&inode->i_flctx->flc_lease))

	if (!list_empty_careful(&flctx->flc_lease))

		return __break_lease(inode, mode, FL_LEASE);

	return 0;

}



static inline int break_deleg(struct inode *inode, unsigned int mode)

{

	struct file_lock_context *flctx;



	/*

	 * Since this check is lockless, we must ensure that any refcounts

	 * taken are done before checking i_flctx->flc_lease. Otherwise, we

	 * could end up racing with tasks trying to set a new lease on this

	 * file.

	 */

	flctx = READ_ONCE(inode->i_flctx);

	if (!flctx)

		return 0;

	smp_mb();

	if (inode->i_flctx && !list_empty_careful(&inode->i_flctx->flc_lease))

	if (!list_empty_careful(&flctx->flc_lease))

		return __break_lease(inode, mode, FL_DELEG);

	return 0;

}